The present invention relates to sputtering apparatus and method of a magnetron sputtering system configured to form a thin film on a large substrate using small-sized sputtering electrodes.
Sputtering facilitates formation of thin films of high melting point materials or compounds when compared to vacuum vapor deposition, and is thus widely used in industrial fields which involve processing of semiconductors, electronic components, etc. Magnetron sputtering, in which a magnetic field is formed in the vicinity of a target using permanent magnets or electromagnets, eliminates a disadvantage of sputtering in which it generally takes ten times or more longer to form a thin film than is required for vacuum vapor disposition. Magnetron sputtering thus allows for mass-production of thin films.
An electrode used in the conventional magnetron sputtering will be described with reference to FIGS. 9A, 9B, and 10.
FIG. 9A is a plan view of a conventional magnetron sputtering electrode with a rectangular flat target and FIG. 9B is a sectional view of the electrode taken along a line A-A' of FIG. 9A. FIG. 10 is a perspective view of the target of the electrode. A rectangular flat target 1 is bonded to a backing plate 2 by means of a soldering material such as indium or the like, which is set on a main body 4 via an O ring 3 disposed for the purpose of vacuum sealing. Magnets for magnetron discharge are provided at a rear side of the target 1 to form closed lines of magnetic force 6. The magnets 5 are so arranged that part of the lines of magnetic force 6 become parallel to each other at a front surface of the target 1. Due to this arrangement, on the front surface of the target 1 there is generated a closed tunnel of a toroidal magnetic field 7.
The operational principle of the magnetron sputtering electrode of the above configuration will be explained with reference to FIGS. 10 and 11.
FIG. 11 schematically shows a sputtering apparatus provided with the magnetron sputtering electrode described above.
Referring to FIG. 11, generally, a magnetron sputtering electrode 12 is placed in a vacuum chamber 9 via a sleeve of insulating material 10. When a thin film is to be formed, the vacuum chamber 9 is evacuated by a vacuum pump 13 to a high degree of vacuum, i.e., approximately 10.sup.-7 Torr. Then, a discharge gas such as Ar or the like is introduced from gas cylinder 14 into the vacuum chamber 9 through a flow rate regulator 15 to keep the chamber 9 at about 10.sup.-3 -10.sup.-2 Torr. In this state, when a negative voltage or a high frequency voltage is applied to the sputtering electrode 12, with the target 1 thereon, from a direct current power source or an alternating current power source 11, a magnetron discharge is brought about in the vicinity of the target 1 between an electric field and the toroidal magnetic field 7 formed by the magnets 5 in FIG. 10. As a result, plasma ions collide against the target 1, thereby sputtering the target 1. The sputtered particles are deposited on a substrate 18 set on a holder 17, so that a thin film is obtained on the substrate 18.
When the above-described conventional magnetron sputtering electrode is used, plasma density becomes high at areas where lines of magnetic force running parallel to the surface of the target are most intense. In other words, sputtering proceeds fast at an area 8 in FIG. 10, while other areas of the target 1 are not uniformly sputtered because of re-adhesion of sputtered particles to the target or like reasons. It is accordingly necessary to regulate the size of the target and the distance between magnets or between the target and substrate so as to obtain the desired uniformity in thickness of a thin film formed on a substrate facing the target. In general, it is necessary for the target to have one side which is approximately twice the size of the substrate to secure satisfactory film thickness uniformity.
Meanwhile, apparatuses that employ small sputtering electrodes to form a thin film on a large substrate have been applied to solve the aforementioned problem.
In one example, a plurality of flat magnetron sputtering electrodes are inclined to the large substrate 18 as shown in FIG. 12 to effectively provide film thickness uniformity on the large substrate.
In a different example of sputtering apparatus shown in FIG. 13, two kinds of targets, namely, a flat inner target 19 and an outer target 20 with an inclined surface are combined and independently controlled. A magnetic field is formed in the vicinity of the combined target by an inner electromagnetic coil 21 and an outer electromagnetic coil 22. Currents running in the two coils 21, 22 are independently controlled by a magnet power source 23, so that the magnetic field is optimized. At the same time, sputtering power fed to each target is separately controlled by a sputtering power source 11. The apparatus is intended to secure good thickness uniformity of a thin film on the large substrate 18 in this manner.
However, the target is not uniformly sputtered in the prior art apparatuses of FIGS. 12 and 13. FIGS. 14A and 14B show sectional views of the sputtered target in the example of FIG. 12, wherein the shaded parts are sputtered regions. FIG. 14A shows the state at an early stage of sputtering, FIG. 14B being a state immediately before the target becomes unusable. As is clear from FIGS. 14A and 14B, the target is not uniformly sputtered, i.e., it is sputtered differently depending on the position. Even if there remains a large amount of unsputtered regions in the target and the target is still sufficiently thick, the target is nevertheless regarded as useless when the target has locally thin portions. Consequently, the expensive target is poorly utilized.
Due to the changes in the shapes of the eroded/sputtered parts of the targets as in FIGS. 14A and 14B, the angles of incidence of the sputtered particles on the substrate are greatly different. If the power fed to the magnetic field in the vicinity of the target and electrode is kept constant both in the early stage and in the final stage of sputtering, the film thickness uniformity on the substrate is lost.
FIGS. 15A and 15B show the sputtered target in the apparatus of FIG. 13. A shaded part similarly represents a sputtered region. FIG. 15A shows the state in the early stage of sputtering, while FIG. 15B shows a state immediately before the target becomes useless.
Although the erosion of the target by sputtering also progresses considerably differently in the early stage and in the final stage in the arrangement of FIG. 13, if the currents in the coils are controlled by the magnet power source 23, the magnetic field is adjusted, whereby the film thickness uniformity can be maintained. However, the above adjustment of the magnetic field requires a complicated control means.
While the thickness uniformity can be maintained despite changes of the target with time, there is another problem in that physical properties of thin films, e.g., composition and structure of thin films are not uniform among the production lots and substrates in the case of sputtering of compounds or according to a reactive sputtering method.